As one of the major renewable energy sources, solar energy has the potential to become an essential component of future global energy production. Dye-sensitized solar cells (DSSCs), represent one of the most promising of several alternative, cost-effective concepts for solar-to-electric energy conversion that has been offered over the past decade to challenge conventional silicon solar cells. The conventional configuration of a DSSC consists of a sintered, wide bandgap semiconductor TiO2 nanoparticle network film, a ruthenium-based dye (i.e., sensitizer), and an electrolyte. Upon the absorption of photons, the dye generates excitons (i.e., electron-hole pairs). Subsequently, the electrons inject into the TiO2 photoanode to generate photocurrent; scavenged by a redox couple, holes transport to the cathode. The performance of a DSSC can be improved by optimizing the semiconductor TiO2 nanoparticle film, sensitizer, and electrolyte. However, much research remains to be done to improve the efficiency and remove the practical problems related to DSSCs. For example, typically, a 10-μm-thick mesoporous TiO2 (anatase nanocrystals) film with a porosity of 50% is employed as an electron-accepting species in a DSSC. The film is prepared by dispersing 15-30 nm colloidal TiO2 particles on a conductive glass support resulting in a network of randomly dispersed nanocrystals. While versatile and robust, these sintered three dimensional TiO2 nanoparticle films lead to enhanced scattering of free electrons and electron trapping at the interfaces, thereby reducing electron mobility and exhibiting less efficient electron transport. The electron and hole transport across several ill-defined, heterogeneous interfaces in TiO2 nanoparticle films is very complex.
In this context, highly ordered, vertically oriented TiO2 nanotube arrays of different aspect ratios and surface qualities have recently been fabricated as alternative nanoscale architectures to substitute the sintered TiO2 nanoparticle films in DSSCs. They are produced by potentiostatic anodization of titanium (Ti) foil or Ti thin films that have been sputtered using radio-frequency (RF) sputter deposition on a variety of substrates with fluorine-containing electrolytes. The nanotubular morphology of these arrays offers a large internal surface area with no concomitant decrease in geometric and structural order. The vertical orientation of the crystalline nanotube arrays makes them excellent electron percolation pathways for efficient, vectorial charge transport along the nanotube axis.
Solar cells produced from TiO2 nanotube arrays show enhanced charge collection efficiency and enhanced light scattering over sintered TiO2 nanoparticle films. The use of TiO2 nanotubes reduces carrier scattering loss and nonradiative recombination by eliminating unnecessary lateral transport (i.e., hopping between TiO2 nanoparticles) and its resulting recombination loss.
Accordingly, it is a primary objective of the present invention to provide a surface treatment for TiO2 photoanodes to increase the loading of sensitizers on the TiO2 surface and also improve the electronic interaction between the sensitizer (i.e., dye) and the TiO2 surface, thereby yielding improved performance. Thus, by using the process of the present invention, a significant step forward in the amount of dye adsorption and the charge transfer from dye molecules to photoanodes is achieved, and, ultimately, an enhanced PCE obtained.
The method of accomplishing this primary objective as well as others will become apparent from the detailed description of the invention which follows hereafter.